In many present-day computer applications it is desirable to interconnect the computer to a variety of peripheral units. These units may include printers, terminals or digital storage units. In addition, it may also be desirable to interconnect two or more computers in a network arrangement either in one location or in remote locations. For example, it may be desirable to connect over an ordinary telephone line either two computers or a computer and a remote terminal. In order to accomplish this, a data "link" is established over the telephone line by means of peripheral circuits known as "modems". Each modem converts the digital signals produced by its associated computer into audio-frequency signals which can then be transmitted over the telephone line. The corresponding modem at the opposite end of the telephone line receives the audio-frequency signals and reconverts them into the digital signals which are then used by its associated computer or terminal.
In order to operate with a computer system, a peripheral circuit must convert its internal electronic signals into a set of electronic signals which can be properly received and processed by the computer. Well-known prior art peripheral circuits typically communicate with their associated computer by means of a "standard" set of electronic signals which have been established for electronic communications between computer units. Once such standard signal set is known as "RS232" standard interface signals in which a digital "0" and "1" are represented by plus or minus twelve volt signals, respectively.
In order to operate with RS232 signal levels, a computer must have its own interface circuitry to convert the RS232 voltage levels into the internal voltage levels actually used in the machine. In many computers, a special RS232 interface or "port" is provided for use with peripheral units such as printers, terminals or modems.
A problem has arisen with many of the smaller, relatively inexpensive "personal" computers because these computers are not typically provided with a RS232 port. Therefore, these smaller computers have either not been able to interface with prior art peripherals or, in order to interface with such peripherals, a special RS232 interface had to be purchased by the computer user at an additional cost of several hundred dollars. Often this additional cost made the use of a particular peripheral impractical with such computers.
It is therefore an object of the invention to provide an interface which will communicate with computers having an RS232 interface and with computers which do not have RS232 interface.
It is a further object of the invention to provide a dual interface using a minimum amount of circuitry and components.
It is a still further object of the invention to produce a dual interface in which the output signals produced by the interface may be rapidly and easily switched between one standard set of signals and another.